A MAF sensor is one of a number of different sensors which provide various input data for an engine electronic control unit (ECU) which processes data for controlling various aspects of operation of an internal combustion engine.
Although calibration of a new MAF sensor may be performed at time of manufacture under controlled conditions, various factors which can potentially affect accuracy of initial calibration may nonetheless be present. Accuracy of calibration therefore depends on the accuracy with which such factors are controlled.
When a MAF sensor is in use, various factors may have an effect on MAF sensor calibration which causes the MAF sensor to lose accuracy.
Examples of factors which may affect calibration include set-up variables encountered when a MAF sensor is bench-tested, flow turbulence, aging of materials, etc. Loss of proper calibration may also go unnoticed unless calibration is re-checked.
The precision with which an engine's operation is controlled is relevant to the engine's compliance with certain requirements, such as tailpipe emission regulations. Because control precision depends on accuracy of input data to the engine ECU, any sensor which looses accuracy has the potential to degrade the precision with which the ECU controls an engine.
A known control strategy for an engine's EGR system operates as follows.
The engine ECU calculates total mass flow passing through an intake manifold to engine cylinders by using engine speed, a calculated density of the flow, and engine volumetric efficiency. The flow comprises a mixture of recirculated exhaust and fresh air which has entered and passed through a first portion of an intake system upstream of a mixing point where the exhaust entrains with the fresh air.
The calculated density is premised on the assumption that the flow behaves as an ideal gas and is calculated using intake manifold pressure and intake manifold temperature.
EGR is defined as a percentage (or fraction) of the total mass flow. The remainder is therefore fresh air whose mass flow has been measured by an output signal of a MAF sensor in the first portion of the intake system.
The strategy is effective over a range of engine speeds and engine loads to cause EGR to be controlled to a target EGR percentage which can change as the engine operates. Subtracting the target EGR percentage from unity yields a target percentage for fresh air mass flow. The strategy controls the engine to cause the output signal of the MAF sensor to faithfully follow the fresh air mass flow target percentage. The strategy may control modulation of EGR flow and/or modulation of the total flow entering the intake manifold.